1. Field of the Invention
This invention relates generally to the field of immunology and specifically to a chemokine binding protein encoded by a variety of poxviruses and methods of use therefor.
2. Description of Related Art
It is becoming increasingly clear that viruses which make their living within cells of higher-order vertebrates must have evolved to specifically avoid the host immune system (Gooding, L., Cell, 91:5-7, 1992; Marrack, P. and Kappler, J., Cell, 76:323-332, 1994; Smith, G., Trends in Micro., 82:80-88, 1994). In fact, virus survival is dependent upon strategies which can evade, suppress, counteract, or otherwise confound the myriad of host responses to a foreign invader. The selection pressure conferred by the effector arms of the immune system can clearly be a powerful element of evolutionary pressure, and all eukaryotic viruses existing today contain imprints or remnants of their battles with the immune system, either as encoded proteins or as evidenced by their particular biological survival strategies.
The larger DNA viruses (i.e. the adenoviruses, herpesviruses, iridoviruses and poxviruses) specifically encode proteins that function to protect the virus from immune recognition and/or clearance by the infected host. Such xe2x80x9csubversivexe2x80x9d viral proteins are now providing information concerning the functional operations of the immune system, and it is likely that many more discoveries of new members of this growing family will be identified in the future.
In the 1980""s the term xe2x80x9cvirokinexe2x80x9d was proposed to describe virus-encoded proteins secreted from infected cells which function by mimicking extracellular signaling molecules such as cytokines or other secreted regulators important for the host immune repertoire (Kotwal, G. and Moss, B., Nature, 335:176-178, 1988). Later, in the 1990""s the term xe2x80x9cviroceptorxe2x80x9d was introduced to account for the observation that some virus encoded proteins that mimic important cellular receptors and function by diverting host cytokines away from their normal receptors, thus interrupting the immune circuitry at its earliest stages (Upton, et al., Virology, 184:370, 1991; Schreiber and McFadden, Virology, 204:692-705, 1994).
Recent studies on a particular poxvirus, myxoma virus, have shown that the virus disrupts the immune system by a variety of strategies (McFadden and Graham, Seminars in Virology, 5:421-429, 1994). Myxoma virus is the infectious agent of a virulent systemic disease of domestic rabbits called myxomatosis. Originally described in the last century, myxoma was the first virus pathogen discovered for a laboratory animal and was the first viral agent ever deliberately introduced into the environment for the explicit purpose of pest eradication. Since its release into the Australian and European feral rabbit populations more than 40 years ago, the field strains of both the rabbit and virus have been subjected to mutual evolutionary and selective pressures that have resulted in a steady-state enzootic in the inoculated areas (Fenner, F. and Ratcliffe, F. N., xe2x80x9cMyxomatosisxe2x80x9d, Cambridge University Press, London, 1965).
Myxoma shares many of the biologic features associated with other poxviruses, namely cytoplasmic location of replication and a large double stranded DNA genome (160 kilobases). Multiple lines of evidence indicate that myxoma, like all poxviruses, encodes multiple gene products whose function is to permit the spread and propagation of the virus in a variety of host tissues. Some of these viral proteins specifically counteract or subvert the development of the host inflammatory response and acquired cellular immunity, and poxviruses in general have been a rich source of such immunomodulatory proteins (Turner, P. C., and Moyer, R. W., Cur. Top. Microbiol. Imm., 163:125-152, 1990; Buller, R. M. L., and Palumbo, G. J., Micro. Dev., 55:80-122, 1991; Smith, G. L., J., Gen. Virol., 94:1725-1740, 1993; McFadden, G., (Ed.), xe2x80x9cViroceptors, virokines and related immune modulators encoded by DNA virusesxe2x80x9d, R. G. Landes Co., Austin Tex., 1995).
Examples of such immunomodulatory gene products include myxoma growth factor (MGF), which stimulates neighboring cells in a paracrine-like fashion via the cellular epidermal growth factor receptor (Upton, et al., J. Virol., 61:1271-1275, 1987; Opgenorth, et al., Virol., 186:185-191, 1992; Opgenorth, et al., Virol., 192:701-708, 1992; Opgenorth, et al., J Virol., 66:4720-4731, 1992); Serp 1, a secreted glycoprotein with serine protease inhibitor activity, that prevents development of the early inflammatory response (Upton, et al., Virol., 179:628-631, 1990; Lomas, et al., JBC, 268:516-521, 1993; Macen, et al., Virol., 195:348-363, 1993); T2, a secreted viral homologue of the cellular tumor necrosis factor (TNF) receptor superfamily, that binds and inhibits rabbit TNF (Smith, et al., BBRC, 176:335-342, 1991; Schreiber, M. and McFadden, G., supra, 1994; Upton, et al., supra, 1991); T7, a secreted viral homologue of the cellular interferon-xcex3 receptor, that binds and inhibits rabbit interferon-xcex3 (Upton, et al., Science, 258:1369, 1992; Upton and McFadden, Methods in Molecular Genetics, 4:383, 1994; Mossman, et al., In: xe2x80x9cViroceptors, virokines and related immune modulatorsxe2x80x9d p. 41-54 Ed. McFadden, R. G. Landers, Co., 1995); and M11L, a surface receptor-like protein that interferes within the inflammatory response by an unknown mechanism (Opgenorth, et al., supra; Graham, et al., Virol, 191:112-124, 1992);
Immunomodulatory proteins also include chemotactic cytokines, called xe2x80x9cchemokinesxe2x80x9d. Chemokines are small molecular weight immune ligands which are chemoattractants for leukocytes, such as especially neutrophils, basophils, monocytes and T cells. There are two major classes of chemokines which both contain four conserved cysteine residues which form disulfide bonds in the tertiary structure of the proteins. The xcex1 class is designated Cxe2x80x94Xxe2x80x94C (where X is any amino acid), which includes IL-8, CTAP-III, gro/MGSA and ENA-78; and the xcex2 class, designated Cxe2x80x94C, which includes MCP-1, MIP-1xcex1 and xcex2, and regulated on activation, normal T expressed and secreted protein (RANTES). The designations of the classes are according to whether an intervening residue spaces the first two cysteines in the motif. In general, most Cxe2x80x94Xxe2x80x94C chemokines are chemoattractants for neutrophils but not monocytes, whereas Cxe2x80x94C chemokines appear to attract monocytes but not neutrophils. Recently, a third group of chemokines, the xe2x80x9cCxe2x80x9d group, was designated by the discovery of a new protein called lymphotactin (Kelner, et al., Science, 266:1395-1933, 1994). The chemokine family is believed to be critically important in the infiltration of lymphocytes and monocytes into sites of inflammation.
It is highly likely that more immunomodulatory viral genes remain to be discovered. Not only will these and related gene products provide useful tools to dissect out the different arms of the host antiviral defense mechanisms, but they may also provide new probes to identify novel elements of the cellular immune repertoire and new classes of drugs to suppress inflammation and dysregulation of the immune system.
The present invention describes an unexpectedly discovered novel soluble virus-specific inhibitor for a class of cytokines which are involved in leukocyte chemotaxis and are collectively referred to as xe2x80x9cchemokinesxe2x80x9d. The protein of the invention is a Type I xe2x80x9cchemokine binding proteinxe2x80x9d (CBP-I) and is a gene product of the T7 gene of myxoma virus, which previously had been shown to encode the interferon-xcex3 (IFN-xcex3) receptor homologue. However, in sharp contrast to the extreme specificity of the T7 gene product for the rabbit ligand (IFN-xcex3), the CBP-I of the invention binds very well to mouse and human chemokines. The CBP-I and related homologues encoded by other poxviruses is useful for treatment of a variety of inflammatory disorders in which excessive influx of leukocytes is associated with the pathogenic process.